1. A hematopoietic colony forming assay that detects transformation of lymphoid cells was utilized to demonstrate that BALB- and Harvey-MSV transform a novel lymphoid progenitor cell both in vitro and in vivo. 2. The ST strain of feline sarcoma virus (ST-FeSV) was shown to transform murine hematopoietic cells of the pre-B-cell phenotype in vitro. 3. Deletion mutants of Abelson-MuLV were constructed and utilized to locate the region of the viral genome that is required for fibroblast and lymphoid cell transformation as well as the ability to induce disease in infected animals.